Engines operating on gaseous fuels, such as natural gas, are commonly supplied with a lean fuel mixture, which is a mixture of air and fuel containing a relatively high ratio of air to fuel. The lean fuel mixture often results in misfires, detonation, incomplete combustion and poor fuel economy. One factor that can lead to such events is the poor ability of conventional spark plugs to effectively ignite a lean fuel mixture in the cylinder of the operating engine. More effective combustion of lean fuel mixtures can be achieved using a pre-combustion chamber.
Pre-chamber (i.e., pre-combustion chamber) spark plugs are typically used to enhance the flammability limits in engines such as natural gas burn engines. As in any spark plug, the pre-chamber spark plugs typically include a pair of electrodes including a ground electrode and a charged electrode. The ground electrode is annular shaped with the a plurality of radially inward projecting tips that surround the charged electrode. Consequently, the charged electrode generally extends axially into the charged electrode generally along a central axis defined by the annular ground electrode.
To extend spark plug life, the tips generally include a precious metal (PM) material secured to a base material. While such inclusion extends the life of the spark plug, spark initiation and maintenance can be an issue. Specifically, it has been determined by the applicant that the shape and configuration of the base material to which the PM material is mounted relative to the shape of the PM material can affect the spark initiation and maintenance between the ground and charged electrode. That is, when a non-homogeneous sparking surface is present, sparks can occur randomly in various locations which results in a large coefficient of variation in indicated mean effective pressure (C0V IMEP). This non-homogeneous sparking surface can also be generated by the shape of the base material (i.e. lobes or spokes) to which the PM material is mounted. If the base material is exposed to or close enough to the charged electrode, the surface of the base material forms part of the sparking surface generating a non-homogenous sparking surface.
As such, there is a need in the art for a pre-chamber spark plug that has extended life enabled by the inclusion of PM material, but that does not suffer from spark initiation and maintenance issues resulting from non-homogenous sparking surfaces present in the art. Embodiments of the present invention provides such improvements in pre-chamber spark plugs and particularly ground electrodes for pre-chamber spark plugs.